The present invention is directed, in general, to the field of seismic exploration and seismology and, more particularly, to seismic sensors such as geophone assemblies. It also encompasses a method and apparatus for compensating the effect of gravity on seismic sensors as well as a method and apparatus to overcome the bandwidth limitations of geophones towards long wavelengths.
3-component seismometers are being used in geophysics to explore structures below the earth's crust. Whereas the vertical alignment of a seismometer can be easily accomplished on land, this calls for elaborate mechanical and lately electronic appliances on the seabed.
Seismometers often contain geophones as sensors. In principle, an electrodynamic geophone comprises a coil, a magnet and a suspension, which allows the coil to move only in one direction relative to the magnet and the magnetic field is directed in such a way that said movement induces a current in the coil.
Depending on the construction most often the magnet or the coil is attached to an enclosure, which is coupled to the area under investigation. Due to the inertia of the moving part a current is induced in the coil, which is proportional to the ground motion. Common constructions are disclosed in U.S. Pat. No. 3,718,900 and U.S. Pat. No. 4,152,692.
Different types of geophones are used for horizontal and for vertical measurements relative to the direction of gravity. The vertical geophone compensates the influence of gravity by pre-stressing the suspension of the moving part. A horizontal geophone does without pre-stress.
In order to detect ground movements in all directions, one vertical geophone and two horizontal geophones at a time are assembled into a three component seismometer such that vertical, north-southern, and east-western ground motions can be detected independently from each other after appropriate leveling. Such arrangements have been used in prospection geophysics for many years. An alternative assembly that is mainly used for broadband seismology only comprises one single type of geophone and became known as “Galperin arrangement”. A drawback for earthquake geophysics (seismology) is the fact that below the eigenfrequency of the moving part the sensitivity of a geophone declines rapidly. The eigenfrequency of manufactured geophones is in the range of 1 to 20 Hz. Seismology studies ground motions from 1 milliHz to 20 Hz. Therefore, geophones for prospection geophysics are unsuitable for seismology.
A paper by E. Wielandt, “Design Principles of Electronic Inertial Seismometers”, published in “Earthquakes: Observation, Theory and Interpretation”, 1983, LXXXV Corso, Soc. Italiana di Fisica, Bologna, describes arrangements that detect ground motions in the seismological bandwidth. All contemporary so called “broadband seismometers” have in common that the spring mounted seismic mass is kept at rest relative to the seismometer's enclosure due to feedback by means of a position detector and an electrodynamic tracking arrangement, i.e. the seismic mass closely follows the ground motion. Thus the influence of the mechanical eigenfrequency on the propagation response is nullified. The measurement signal itself is derived from the feedback signal, which is proportional to the ground motion in the seismological bandwidth as well. An elaborate implementation is disclosed in U.S. Pat. No. 4,280,206.
U.S. Pat. No. 6,075,754 discloses a simple version of a similar arrangement that uses a standard geophone as the seismic mass, as acceleration detector, and as tracking means all at the same time.
The search for oil and gas is extended to deeper and deeper sea areas due to the continuing depletion of terrestrial and coastal reservoirs. The high cost of deep water operations asks for an increasing quality of seismic exploration and therefore, more and more often seismometers are deployed on the seabed in addition to the traditional registration of pressure waves using hydrophones on the water surface. Usually, mechanical arrangements, e.g. gimballed suspensions damped by high-viscosity fluids, are used for the necessary vertical alignment. The horizontal north-south and east-west orientation is accomplished either by means of an integrated compass or by recording seismic calibration pulses. Also, borehole seismic surveys are riddled by the fact that boreholes are no longer vertical and straight, but full of bends and even horizontal sections. Therefore, the present invention opens up new possibilities for seismic borehole sensors.
As early as 1998 EPRO GmbH, Germany in a product catalogue for “Vibration Transducer PR9268” describes that the coil of a transducer that has been mounted in a tilted position may be brought back into its operating position by injecting a suitable electrical compensation current, and U.S. Pat. No. 6,412,592 B1 discloses how such a compensation current can be automatically generated depending on the tilt angle by means of a potentiometer coupled to the geophone mechanically.
Document WO 03/096071 A1 discloses a method and arrangement to reduce the measurement error, which is due to the tilt of a geophone by feeding a compensation current into the geophone's coil that has been derived from a separate acceleration or tilt sensor. Thereby the residual error under all tilt conditions is diminished to an error that amounts to a maximum tilt angle of 10 degrees. Thus this method respectively this apparatus is suitable to compensate the tilt of geophones down to an eigenfrequency of 10 Hz without calibration.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
It is the object of the present invention to specify a method and an apparatus of the aforementioned nature that fully compensates any tilt without calibration even for geophones under 10 Hz eigenfrequency using less energy than the arrangement disclosed in WO 03/096071 A1.